This invention relates to a roller for running a flat belt such as a photosensitive belt, transcription belt, etc. of an electrophotographic machine.
In an electrophotographic machine, for example, it has been known that in order to cut down in weight and size of the machine, a flat belt, formed at its surface by a photosensitive layer or dielectric layer, for training among a plurality of rollers are arranged in substantially parallel with one another is used as a photosensitive belt or a transcription belt, etc. instead of a transcription drum or photosensitive drum.
In this case, the roller for running the flat belt has following problems for forming a accurate image.
(1) When a foreign particle enters between the flat belt and roller, the flat belt may be scratched or dented to cause partial dropping out of an image.
When a foreign particle such as dust, toner, carrier enters between the belt and roller, the roller a surface layer of which is made of metal is liable to be scratched. Since the foreign particle is only put on the surface of the roller, it can be removed easily. However, sever scratches or dents may be caused thereon without buffer function. On the other hand, in case where the surface layer of the roller is made of an elastic material such as a rubber, since the foreign particle is buried on the surface layer, little scratch or dent may be caused. However, it is not so easy to remove the foreign particle, thus the scratches or dents shall be caused repeatedly.
(2) A deviation of the flat belt to one side leads to a breakage of a side end of the belt.
As most of the flat belts used as a photosensitive belt, transcription belt, etc. are made of a material with less ductility and high strength, such as a plastic film, metal leaf. Hence, with such less deformability, the belt cannot absorb a dimensional error in each related part, mounting error of the roller, imbalance of the belt tension, uniformity of the peripheral length of the belt, and so on. As a result, a deviation force which deviates the belt to one side in an axial direction affects during belt running.
In order to prevent the belt deviation, there have been techniques disclosed in Japanese Patent Application Laying Open Gazette No. 56-127501 and 59-205052 in which a guide for preventing the deviation is provided at the belt, and a technique disclosed in Japanese Patent Application Laying Open Gazette No. 57-60347 in which the belt deviation is ceased by a restriction member provided at the roller or vicinity thereof. Nevertheless, when the deviation force increases, the side end of the belt and restriction members may break down. Therefore it has been desired that the roller is made so as no to cause the belt deviation, and even if the belt deviation is caused, so as to easily regulate the deviation.
In order to prevent the belt deviation, it is required to decrease a friction coefficient between the flat belt and roller and the deviation force during belt running (the little deviation force leads to an easy regulation of the deviation). It is further required to maintain stable low friction coefficient independent from temperature and humidity.
(3) A waving flat belt leads to distortion on the image or partial blurred image.
A thin flat belt such as a photosensitive belt used in an electrophotographic machine is apt to be buckled in a width direction to be waved when the flat belt receives large lateral force.
It is desired that the above problems are solved not only in an electrophotographic machine but also in a device requiring an accurate running of the flat belt.
It is therefore an object of the present invention to provide a roller for accurately running a flat belt with a simple construction and a flat belt driving device having the roller which can solve the above problems by preventing the scratches and dents thereat and waving thereof and by decreasing the deviation force during belt running.